Alternative diagnostic diagrams and the "forgotten" population of weak line galaxies in the SDSS

Abstract: A numerous population of weak line galaxies (WLGs) is often left out of statistical studies on emission line galaxies (ELGs) due to the absence of an adequate classification scheme, since classical diagnostic diagrams, like [OIII]/Hb vs [NII]/Ha (the BPT diagram), require the measurement of at least 4 emission lines. This paper aims to remedy this situation by transposing the usual divisory lines between Star Forming (SF) and AGN hosts, and between Seyferts and LINERs to diagrams that are more economical in terms of line quality requirements. By doing this, we rescue from the classification limbo a substantial number of sources and modify the global census of ELGs. More specifically: (1) We use the SDSS DR7 to constitute a suitable sample of 280k ELGs, 1/3 of which are WLGs. (2) Galaxies with strong emission lines are classified using the widely applied criteria of Kewley et al (2001), Kauffmann et al (2003), Stasinska et al (2006) and Kewley et al (2006). (3) We transpose these classification schemes to alternative diagrams keeping [NII]/Ha as a horizontal axis, but replacing Hb by a stronger line (Ha or [OII]), or substituting [OIII]/Hb ratio with the equivalent width of Ha. Optimized equations for the transposed divisory lines are provided. (4) We show that nothing significant is lost in the translation, but that the new diagrams allow one to classify up to 50% more ELGs. (5) Introducing WLGs in the census of galaxies in the local Universe increases the proportion of metal-rich SF galaxies and especially LINERs. (abridged)

• The paper introduces innovative diagnostic diagrams that incorporate overlooked weak line galaxies into SDSS spectral classifications.
• The methodology substitutes traditional emission lines with alternatives, such as using Hα in place of Hβ and exploiting equivalent width metrics, thereby expanding the classifiable sample.
• The findings challenge conventional AGN categorizations by revealing that many weak line galaxies are LINERs or retired systems, which significantly impacts models of galaxy evolution.

An Examination of Weak Line Galaxies within the Sloan Digital Sky Survey

The study by Cid Fernandes and collaborators addresses a significant oversight in the classification of emission line galaxies (ELGs) by focusing on the overlooked population of weak line galaxies (WLGs) within the Sloan Digital Sky Survey (SDSS). Traditional diagnostic diagrams like the BPT diagram (Baldwin, Phillips & Terlevich) rely on the detection of four specific emission lines for spectral classification. Many galaxies with weak lines have been neglected in conventional studies due to the reliance on high-quality line measurements. This paper introduces alternative diagnostic diagrams that enable the inclusion of WLGs, hence providing a more comprehensive classification of ELGs.

Methodological Overview

The authors present a detailed analysis using SDSS Data Release 7, examining a substantial sample of 280,000 ELGs of which approximately one third are WLGs. These WLGs, neglected in traditional emission line studies, are brought into the classification spectrum using alternative diagnostic diagrams that require fewer detectable lines, offering a significant improvement by increasing the classifiable population by up to 50%.

To achieve this, the paper proposes transposing existing theoretical divisory lines between Star Forming (SF) galaxies and Active Galactic Nuclei (AGN) hosts (including Seyferts and LINERs) to alternative diagrams using:

• The BPT$\alpha$ Diagram: Replacing Hβ with Hα for situations with weak Hβ emission.
• The BPTo2 Diagram: Utilizing the O/[O II] versus N/[N II] ratio, leveraging O as a more robust line.
• The EW$\alpha$n2 Diagram: Introducing the equivalent width of Hα, optimizing separations between Seyfert and LINER galaxies.

Results and Implications

The replacement of weaker lines with stronger alternatives or equivalent width metrics allows a broader classification of galaxies. The results highlighted several key findings:

1. WLGs' Classification: WL-H (weak Hβ) galaxies are predominantly LINERs, while WL-O galaxies often represent metal-rich SF galaxies. Few WLGs exhibit Seyfert-like spectra.
2. Enhanced Census: The inclusion of WLGs leads to a revised distribution of galaxy types, showing a notable increase in the number of metal-rich SF galaxies and especially LINERs, thereby altering the local Universe's emission line galaxy census.
3. Diagnostic Efficiency: The BPTo2 and the EW$\alpha$n2 diagrams emerged as notably effective for distinguishing between different galaxy classes, specifically in discriminating LINERs from Seyferts with higher confidence than traditional methods.
4. Seyfert/LINER Dichotomy: They investigate this division further, suggesting that much of what classifies as LINERs could overlap significantly with galaxies that have retired from star formation, challenging traditional AGN classification.

Conclusion and Future Directions

This paper reshapes the landscape of spectral classification for emission line galaxies by emphasizing the inclusion of WLGs using novel diagnostic methods. It urges the re-evaluation of the class dichotomies and stresses the importance of considering alternative ionizing sources, such as old stellar populations in retired galaxies. The implications of this study extend to the refinement of galaxy evolution models and the interpretation of emission lines in broader astrophysical contexts.

Future work may aim to validate these alternative classification schemes with other large datasets or in high-redshift regimes. Furthermore, understanding the physical underpinnings of LINER-like emissions in retired galaxies presents an exciting avenue for research, with potential implications for our comprehension of galaxy lifecycle and evolution.